1. Field of the Invention
The present invention relates to an improved method for heat treating a mold cast product, particularly a camshaft.
2. Description of the Related Art
In conventional mold casting, a cast product is cooled deep inside while it is held in a mold, during which cooling process a constraining force of the mold is applied to the cast product, thereby causing cracks to arise in the cast product. A measure to prevent such cracking is proposed in, for example, Japanese Patent Kokoku (Post-Exam) Publication No. HEI-5-45347 disclosing a method for mold casting a workpiece comprising a camshaft. In the proposed casting method, after molten metal is poured into a mold, that part of the molten metal which is held in contact with the mold is rapidly cooled or quenched so that a shell-like solidified layer or skin is formed on the surface of the molten metal, whereafter the resulted cast workpiece is released from the mold. Since the workpiece is released from the mold with its inside kept in an unsolidified state, the workpiece is freed from its cracking. In addition, since the workpiece is released from the mold before its inside solidifies, the relevant production cycle time is shortened compared to that of the conventional mold casting, thereby increasing the productivity.
The proposed mold casting method is efficient and enables production of workpieces of excellent shape. However, the resulted cast products need to be subjected to additional heat treatment (hardening and tempering) so as to impart abrasion-resistive property and toughness thereto. This heat treatment includes three different heating processes, namely, a first heating for pre-casting melting, a second heating for hardening and a third heating for tempering, thereby requiring an increased amount of heat energy and increasing the number of production processes.
In the proposed casting method, the workpiece is hardened in its entirety. However, when the workpiece is a camshaft, a journal portion thereof should desirably be kept unhardened. Thus, in certain uses, the workpiece must have a hardened portion and a unhardened portion at the same time but this can hardly be achieved by the proposed casting method.
Consequently, there is a need to provide mold casting in which measures are taken to save energy and reduce the number of the required processes. In addition, there is a need to provide a technique which enables the coexistence of a hardened portion and a unhardened portion in a single workpiece.
It is already known to harden a camshaft by induction heating the camshaft to a hardening temperature and then soaking the heated camshaft into a cooling agent or emitting a jet of the cooling agent thereat. However, from the standpoint of energy conservation, it is more desirable to instantly forcedly cool such a product produced by the mold casting as proposed in Japanese Patent Publication No. 5-45347.
An elongate workpiece such as a camshaft is liable to be bent easily. Thus, one may propose rapidly cooling the workpiece while restraining the workpiece by means of an appropriate jig. However, since such an elongate workpiece shrinks substantially upon rapid cooling, the restraint by the jig may possibly interrupt the shrinkage, thereby causing cracks to arise in the workpiece.
Accordingly, there is a need for a technique which enables forced cooling of the workpiece and energy saving while preventing cracking of the workpiece.
In the mold casting of the above-described publication, the cast product is forcedly cooled in the mold. At this time, it is difficult to minutely control the cooling speed, because the mold has a large heat capacity.
Further, where the cast product is a camshaft, a cam portion of the camshaft needs to be hardened to increase the abrasion-resistivity thereof while a journal portion of the camshaft needs to be machined and thus should not be hardened. That is, a certain cast product needs to have a hardened portion and an unhardened portion at the same time. However, in the method of the above-described publication, although hardening of the entirety of the camshaft is possible, it is not possible to minutely control the cooling operation so that an unhardened portion can coexist with a hardened portion. Consequently, there is a need for a heat treatment method which enables the coexistence of a hardened potion and an unhardened portion in a single cast product.
To meet its use requirement, the camshaft produced by the method of Japanese Patent Laid-Open Publication No. HEI-5-45347 needs to have a hardened portion while a journal portion thereof should be kept unhardened, as described above. In providing such a product, it has been the conventional practice to mask the journal portion and then spray a cooling agent at the camshaft to thereby harden only the cam portion.
However, such a method in which the journal portion is covered with a masking of desired shape to prevent the journal portion from being sprayed with the cooling agent for hardening does not allow the required temperature control of the journal portion. Without such temperature control, there is a fear that rapid cooling or quenching of the adjoining cam portion will also quench the journal portion, thereby hardening the latter, This makes the relevant hardening operation more difficult to achieve and the range of control of the hardening operation narrower. Consequently, it is required to broaden the range of control of the hardening operation by controlling the cooling speed of a portion desired to be left unhardened.
According to a first aspect of the present invention, there is provided a heat treating method which comprises the steps of: pouring molten metal for iron-based parts into a mold to produce the cast product having a surface layer; releasing the cast product from the mold when the cast product surface layer held in contact with the mold is in a temperature range allowing hardening and became a shell-shaped solidified layer; and hardening the cast product released from the mold and having a hardening allowing temperature, by cooling the cast product with a cooling agent.
In this method, heat remaining in the product after release from the mold is used to harden the product, whereby heat for hardening becomes unnecessary. Consequently, it becomes unnecessary to establish a process for transporting the product to a hardening furnace, thus decreasing the number of required man hours.
Cooling of the mold cast product may be performed by spraying the cooling agent onto the product or soaking the product into the cooling agent.
Preferably, the cooling is performed by spraying the cooling agent locally onto part of the mold-released cast product where hardness is required. This makes it possible to provide a hardened part and an unhardened part in the product so that the hardened part has an abrasion-resistive property and increased toughness while the unhardened part has flexibility.
In a preferred form, the method further comprises the step of pre-cooling part of the mold-released cast product where hardness is not required. At this time, the hardening step may comprise spraying the cooling agent locally onto part of the mold-released cast product where hardness is required. This makes it possible to achieve the coexistence of a hardened portion and an unhardened portion in the product. In the pre-cooling step, the hardness required part of the mold cast product may be maintained at a temperature higher than an A1 transformation point while the hardness non-required part of the mold cast product may be cooled to a temperature lower than the A1 transformation point. By thus cooling the hardness non-required part to a temperature lower than the A1 transformation point, when the product is rehardened, the hardness required part will be hardened with the hardness non-required part remaining unhardened. It thus becomes possible to keep the hardness of the hardness non-required part to a minimum.
After the product releasing from the mold, the product may be restruck to correct a shape thereof and then subjected to forced rapid cooling done by spraying the cooling agent onto the cast product or by soaking the latter into the cooling agent while repeating alternate constraining and non-constraining of the product. Since the shape correction of the product is effected before hardening, a workpiece of excellent shape can be provided for hardening. Cracking of the resulting product can be avoided, because constraining in which the product is corrected in its deformation and bend while being held in a constrained state and non-constraining in which the product is left unconstrained to allow shrinkage are alternated during the hardening.
The alternate constraining and non-constraining of the cast product may be repeated until the temperature of the cast product reaches a martensitic transformation starting temperature (Ms point), whereafter the cast product may be held in a non-constrained state. The Ms point is approximately 180xc2x0 C. Thermal deformation of the product below that temperature is subtle and hence the product does not need to be constrained any more. Below the Ms point, austenite transforms into martensite to thereby cause metallurgical expansion. Accordingly, it is desirable to disconstrain the product so as not to interrupt the transformational expansion.
It is preferable that the method also includes the step of interrupting the cooling when the temperature of the cast product surface layer drops to below a martensitic transformation starting temperature (Ms point) while the temperature of the inside of the product remains higher than the temperature of the product surface layer, so that self tempering of the product surface layer can be effected by an internal residual heat of the cast product. That is, low temperature tempering is effected by using the internal residual heat of the cooled product. As a result, it becomes possible to omit heating for tempering, thereby reducing the number of required man hours. Conventionally, heating has been required at three different occasions but only one time heating is required in the inventive method, thus contributing to energy conservation.
The self tempering may be effected by interrupting the cooling when the temperature of the product surface layer drops to below a martensitic transformation starting temperature (Ms point) and while the temperature of a cooling agent unsprayed part and the inside of a cooling agent sprayed part of the product remains higher than the temperature of the product surface layer, so that the hardened part can be self tempered by residual heat of the cooling agent unsprayed part and internal residual heat of the cooling agent sprayed part.
Desirably, the hardening step comprises spraying onto the cast product the cooling agent in the form of mist resulted from mixing water pressure with air pressure at a given ratio. By simply changing the water pressure to air pressure ratio, cooling rate can be altered, whereby minute cooling control is enabled.
The method may further comprise the step of masking a hardness non-required part of the mold-released cast product. In addition, the hardening step may comprise spraying the cooling agent onto a hardness required part of the product and causing a cooling rate to fall at least once during a drop in temperature of the target of cooling from an A1 transformation point to a martensitic transformation starting temperature (Ms point). By thus masking the part desired not to be hardened, a non-hardened part can be provided easily. Further, by decreasing the cooling rate during the cooling process, an unhardened part may be provided at the masked part. It is also desirable that the hardening comprises intermittently spraying the cooling agent onto the hardness required part and causing a cooling rate to fall at least once during a drop in temperature of the target of cooling from the A1 transformation point to the martensitic transformation starting temperature (Ms) by interrupting the cooling agent spraying.
According to a second aspect of the present invention, there is provided an apparatus for slowly cooling an iron-based part having a hardening allowing temperature, which comprises a plurality of cooling blocks held in contact with a part of the part desired to be slowly cooled. The cooling blocks each comprises: a cooling agent passage for allowing passage of a cooling agent; a recessed portion provided at an outlet of the cooling agent passage; and a porous material member received in the recessed portion so that the cooling agent can be moderately dispersed to thereby partially cool the iron-based part.
In the apparatus thus arranged, part desired to be slowly cooled by means of a cooling agent while controlling the cooling rate, and a part desired to be hardened is forcedly cooled separately. As a result, the resulting product has increased control precision in its entirety. Further, since the cooling agent is moderately dispersed by the porous material member, it becomes possible to cool part of the iron-based part at such a cooling rate that the cooled part does not become hardened.
Each of the cooling blocks may desirably be arranged to serve as a part of a restriking mechanism for correcting a shape of the iron-based part.